Hugo Nebula wrote:
On Sun, 30 Dec 2007 01:04:01 +0000, a particular chimpanzee, The
Natural Philosopher randomly hit the keyboard and produced:

Andy Wade wrote:

Nonsense - you've forgotten to include the surface boundary layer
resistances.

That was straight out of te building regulations hand book.

I tend to trust them more than you.

But he is right. The Resistance value of all the elements is added
together to give the total R-value; a U-value is the reciprocal of
this. The R-value includes the surface resistances of the inside and
outside surfaces (IIRC, between 0.35m�K/W to 0.04m�K/W depending on a
few factors).

Highly dangerous factors. A ripping wind in the loft makes mockery of
them for a start. And reduces rockwool insulation hugely.


However the OP did not ask for the total resistance of this or that
construction, Merely the U values of the materials. That stands as written.

If you want the total U value of a construction do the maths and add in
the boundaries.

BUT as I said, its dangerous to rely on them.

Anyone who has blown on his hands to warm or cool them can tell you that.

Judging by my boiler on times, `I lose 2-4 times more heat in a 20mph
wind...

The difference when I fully boarded out 150mm rockwool in the loft was
immense.

A fan blown heatsink will do about 5-20 times more heatloss than a
convection cooled one. OH its DESIGNED to do that, which houses are not,
but the U value calculations are VERY optimistic if they rely on ANY
boundary layer outside the house, or in a 'cold' loft' whatsoever.